Diffuse reflectance infrared Fourier transform spectroscopy

ABSTRACT

Diffuse reflectance spectroscopy apparatus for use in analyzing a sample comprising a sample receiving location ( 2 ) for receiving a sample ( 3 ) for analysis; an illumination arrangement ( 4 ) for directing light towards a received sample; a detector ( 6 ) for detecting light reflected by a received sample; and collection optics ( 5 ) for directing light reflected by a received sample towards the detector. The illumination arrangement further comprises an interferometer ( 42 ) and a half beam block ( 45   a,    45   b ) which is disposed substantially at a focus in the optical path for blocking light which exits the interferometer, passes said focus, and is reflected from reentering the interferometer. A half beam block ( 45   a ) may be disposed in the optical path between the interferometer and the light source ( 41 ) for blocking light that exits the interferometer back towards the light source and is reflected by the light source from re-entering the interferometer and/or a half beam block ( 45   b ) may be disposed in the optical path on the opposite side of the interferometer than the light source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application and claims thebenefit of priority under 35 U.S.C. § 371 of PCT/GB2014/000286, titledDIFFUSE REFLECTANCE INFRARED FOURIER TRANSFORM SPECTROSCOPY and filed onJul. 14, 2014, which in turn claims priority to GB1312911.9, filed onJul. 18, 2013, the contents of both of which are hereby incorporatedherein by reference in their entireties for all purposes.

This invention relates to diffuse reflectance spectroscopy and inparticular FT-IR (Fourier Transform Infra Red) diffuse reflectancespectroscopy apparatus and methods for manufacturing FT-IR diffusereflectance spectroscopy apparatus.

In the field of reflectance spectroscopy a light beam is shone onto asample. A portion of the light reflected from the sample is collectedand subjected to spectroscopic analysis in order to determine thechemical composition of the sample.

Prediction of the chemical composition of the sample involves the use ofstatistical techniques (known as chemometrics) that relate the opticalenergies detected at various wavelengths to the concentration of aparticular chemical species of interest.

In order to make such predictions, a calibration or model must first bebuilt using standards of known concentration. The process of buildingsuch a model involves the use of many standards, each of which requireanalysis by some other (often wet chemistry) techniques. The alternativeanalysis methods are usually cumbersome and time consuming and thusexpensive. Further, the standards that have been qualified often have alimited shelf life.

It is highly desirable that a calibration set built using one instrumentcan be transferred to another and be effective on a second instrument.It is often the case that small and subtle differences in responsebetween notionally identical instruments cause a second instrument toread somewhat differently to a first when used to analyse the samesample. The best measurement accuracy is achieved by calibration of eachinstrument. However such a practice is often prohibitively expensive.

The more similar in response the instruments can be made, the moreconsistent are the predictions of sample properties.

It would be desirable to provide diffuse reflectance spectroscopyapparatus which are aimed at helping to reduce such variability and moregenerally to help improve performance.

Some particular considerations, taken into account when developing thepresently described apparatus, included:

A tendency for the spectra measured by an apparatus to be dependent onthe positioning of the sample relative to a source of illuminationand/or detection system;

The tendency for differences in illumination between devices and atdifferent times to affect the spectra obtained;

Errors which can be introduced by double modulation artefacts in FTIRspectroscopy;

The fact that there may be differences in background spectra from timeto time and/or possible drifts or malfunctions in operation of theapparatus as a whole.

Various different features of the presently developed and describedapparatus may help in addressing one or more of these issues.

According to a first aspect of the present invention there is provideddiffuse reflectance spectroscopy apparatus for use in analysing a samplecomprising:

a sample receiving location for receiving a sample for analysis;

an illumination arrangement for directing light towards a receivedsample;

a detector for detecting light reflected by a received sample; and

collection optics for directing light reflected by a received sampletowards the detector.

According to a second aspect of the present invention there is provideddiffuse reflectance spectroscopy apparatus for use in analysing a samplecomprising:

a sample receiving location for receiving a sample for analysis; anillumination arrangement for directing light towards a received sample;

a detector for detecting light reflected by a received sample and havinga predetermined light detecting active area; and

collection optics for directing towards the detector, light reflected atan angle within a predetermined angular range by a received sample,wherein the collection optics focus the detector substantially atinfinity.

This helps to avoid variations in the intensity and/or spectrum of lightseen by the detector when the sample height (spacing between the sampleand collection optics) varies. In principle the same reflected rays willbe collected by the collection optics and detected by the detectorirrespective of the sample height within the working limits of thesystem. A change in sample height within the operational range willchange the lateral position of the path of the collected rays throughthe collection system but will not change the rays which are selectedand reach the detector, nor their arrival position at the detector.

One would most naturally focus the detector at the sample receivinglocation. However this causes a problem that the system becomes verysensitive to the sample height (spacing between the sample andcollection optics) because different amounts of light and different raysare collected and are imaged on different parts of the detectordepending on the height.

The illumination arrangement may be arranged for directing anilluminating light beam at a received sample and the optical axis of thecollection optics at the entrance to the collection optics may beinclined to the beam axis of the illuminating light beam.

This can help avoid collection of any specular reflection beam andprovides physical separation between the illumination beam and the inputof the collection optics/the collection beam.

The collection optics may comprise an off axis paraboloid mirror forfocusing light from infinity and an ellipsoid mirror dimensioned tocorrect for aberrations of the paraboloid mirror.

The paraboloid mirror may be arranged in the optical path in thecollection optics before the ellipsoid mirror.

The paraboloid mirror may be the first optical component in thecollection optics.

In an alternative system suitable lenses may be used in place of themirrors.

The size of an entrance pupil of the collection optics may be chosen tobe sufficient to accommodate a predetermined range of sample tocollection optics spacings.

The diameter of the paraboloid mirror may define an entrance pupil ofthe collection optics.

The diameter of the paraboloid mirror may be chosen to be sufficient toaccommodate a predetermined range of sample to collection opticsspacings.

The collection optics may comprise an intermediate focus position and anaperture may be provided at the intermediate focus position and arrangedso that its image at the detector has predetermined dimensions chosen toguard against rays outside a chosen angular range reaching the detector.The predetermined dimensions may be chosen in dependence on thedimensions of the light detecting active area of the detector. Thepredetermined dimensions may be chosen to be substantially the same asthe dimensions of the light detecting active area.

The intermediate focus position may be disposed between the paraboloidmirror and the ellipsoid mirror.

The illumination arrangement may comprise a light source. In other casesthe light source might be provided outside of the illuminationarrangement. The illumination arrangement may be arranged for focusingthe light source substantially at infinity.

According to a third aspect of the present invention there is provideddiffuse reflectance spectroscopy apparatus for use in analysing a samplecomprising:

a sample receiving location for receiving a sample for analysis;

an illumination arrangement for directing light towards a receivedsample;

a detector for detecting light reflected by a received sample; and

collection optics for directing light reflected by a received sampletowards the detector, wherein the illumination arrangement one ofcomprises a light source and is arranged to receive light from a lightsource and the illumination arrangement is arranged for focusing thelight source substantially at infinity.

This will lead to the light from each point on the light source beingspread across the field at the sample receiving location which in turnwill improve uniformity of illumination and consistency in measurements.One might more naturally focus the light source onto the samplereceiving location, but this will lead to more spatial variation inillumination as an image of the light source is formed at the sample,and hence in general more variation in illumination.

The light source may comprise an incandescent lamp. The illuminationarrangement may be arranged for focusing the filament of the lightsource substantially at infinity.

The illumination arrangement may be arranged so that any non-uniformoptical surfaces in the optical path between the light source and thesample location are spaced in the optical path from any image of acarried sample.

The illumination arrangement may comprise an interferometer including abeam splitter and may be arranged so that the image of the sample isspaced in the optical path away from the beam splitter.

The illumination arrangement may comprise an entrance paraboloid mirrorfor collecting light from the light source. The light source may beplaced in the region of the focus of the entrance paraboloid mirror. Inpractice the light source may be placed just beyond the focus of theentrance paraboloid mirror.

The illumination arrangement may be arranged so that the image of thesample will be in the optical path between the entrance paraboloidmirror and the beam splitter.

The illumination arrangement may comprise a J.Stop aperture provided ata focus in the optical path to block rays with an above thresholddivergence.

The illumination arrangement may comprise a relay paraboloid mirrordisposed in the optical path on an opposite side of the interferometerthan the light source. The J.Stop aperture may be provided at the focusof the relay paraboloid mirror.

Note that in alternatives one or more of the interferometer, entranceparaboloid mirror, J.Stop aperture, and relay paraboloid mirror as wellas similar components may be provided outside of the illuminationarrangement of the apparatus.

The illumination arrangement may comprise an interferometer and a halfbeam block which is disposed substantially at a focus in the opticalpath for blocking light which exits the interferometer, passes saidfocus, and is reflected from re-entering the interferometer.

According to a fourth aspect of the present invention there is provideddiffuse reflectance spectroscopy apparatus for use in analysing a samplecomprising:

a sample receiving location for receiving a sample for analysis;

an illumination arrangement for directing light towards a receivedsample;

a detector for detecting light reflected by a received sample; and

collection optics for directing light reflected by a received sampletowards the detector, wherein the illumination arrangement comprises aninterferometer and a half beam block which disposed substantially at afocus in the optical path for blocking light which exits theinterferometer, passes said focus, and is reflected from re-entering theinterferometer.

A half beam block functions because light that is reflected will returnthrough a focus of the system on the opposite side of the optical axisthan it passed in the forward direction. Thus with an obscuration placedhalf way across the beam at a focus half of the light is blocked in theforward direction, and if any of the light that passes is reflected thiswill be blocked when travelling in the reverse direction. This is usefulas it avoids light passing twice through the interferometer and reachingthe detector, that is it avoids double modulated light reaching thedetector.

The half beam block may be disposed in the optical path between theinterferometer and the light source for blocking light that exits theinterferometer back towards the light source and is reflected by thelight source from re-entering the interferometer.

Alternatively, the half beam block may be disposed in the optical pathon the opposite side of the interferometer than the light source.

The illumination arrangement may comprise two half beam blocks, thefirst of which may be disposed in the optical path between theinterferometer and the light source for blocking light that exits theinterferometer back towards the light source and is reflected by thelight source from re-entering the interferometer and the second of whichmay be disposed in the optical path on the opposite side of theinterferometer than the light source.

Where the half beam block is disposed in the optical path between theinterferometer and the light source, the light source may be displacedtransversely from the optical axis towards a side of the beam which isopposite that at which the half beam block is disposed.

The half beam block will have an edge running across the beam. The edgemay be curved such that the unblocked portion of the beam is crescentshaped. At the centre of the beam the edge may be substantially at theoptical axis but at positions away from the centre, the edge may beretracted from the line through the optical axis which would bedescribed by a straight edge.

This can allow more light to enter the system whilst still blockingreflected rays. Aberrations in the system will tend to mean thatreflected light from half a beam allowed through by a half beam blockwill occupy less than half of the initial beam field on return. Thus afull straight edged half beam block is blocking more light thannecessary. Using a curved edged beam block to create a crescent shapedunblocked region increases efficiency by taking the effect of theaberrations into account. The curved edged block can be considered tocompensate for field distortion.

The shape of the edge of half beam block may be chosen to ensure thatsubstantially all rays passing the block in the forward direction will,if reflected, be blocked in the reverse direction. The shape of theblock may be optimised through modelling or empirically for example.

The apparatus may further comprise a reference spectrum acquiringarrangement comprising:

at least one reference sample receiving location; and

a beam switching arrangement having first and second states, the firststate allowing light from the illumination arrangement to reach thesample receiving location and light reflected by a carried sample toreach the collection optics; and

the second state for redirecting light from the illumination arrangementtowards the reference sample receiving location instead of the samplereceiving location and for directing light reflected by a receivedreference sample towards the collection optics allowing selectivedetection at the detector of light reflected by a carried sample andlight reflected by a carried reference sample.

According to a fifth aspect of the present invention there is provideddiffuse reflectance spectroscopy apparatus for use in analysing a samplecomprising:

a sample receiving location for receiving a sample for analysis;

an illumination arrangement for directing light towards a receivedsample;

a detector for detecting light reflected by a received sample; and

collection optics for directing light reflected by a received sampletowards the detector,

wherein the apparatus further comprises a reference spectrum acquiringarrangement comprising:

at least one reference sample receiving location; and

a beam switching arrangement having first and second states,

the first state allowing light from the illumination arrangement toreach the sample receiving location and light reflected by a carriedsample to reach the collection optics; and

the second state for redirecting light from the illumination arrangementtowards the reference sample receiving location instead of the samplereceiving location and for directing light reflected by a receivedreference sample towards the collection optics allowing selectivedetection at the detector of light reflected by a carried sample andlight reflected by a carried reference sample.

This can facilitate the acquisition of a reference spectrum without theuser being required to present the reference sample at the “normal”sample location. Thus for example the reference sample may be storedwithin the apparatus in its operative position. In turn this canencourage the user to take proper reference measurements, or allow theapparatus to take reference measurements automatically without any userinvolvement.

Note that in the above statements the beam switching arrangement simplyallows light to reach the sample and reflected light to reach thecollection in the first state. However it directs light in the secondstate. Thus the beam switching arrangement may in fact play no role indirecting light in the first state—it may be out of the optical path.This is a preferred implementation but is not essential. In otherimplementations the beam switching arrangement might direct light in allstates or direct light in the first state but not the second, forexample. Thus more generally the beam switching arrangement may cause orallow passage of light in the desired directions in each state.

The reference spectrum acquiring arrangement may comprise a referencesample disposed at the reference sample receiving location.

The reference spectrum acquiring arrangement may comprise two referencesample receiving locations. A first may be used for holding a firstreference material for use in acquiring a background spectrum. A secondmay be used for holding a second reference material which is differentfrom the first reference material and is for use in acquiring anoperational check spectrum.

The first reference material will generally be a highly reflective“standard” material. The spectrum of that material is likely to berelatively featureless but the amplitude will change with differences inillumination intensity for example, and differences in illuminationintensity will often be wavelength dependent. Thus a background spectrumacquired using such a sample is useful for normalising spectra acquiredfor other samples to take into account differences in the illuminationcharacteristics at different times, in different apparatus, and so on.

The second reference material will generally be a more featureful butstable “standard” material. A spectrum acquired using the secondreference material can be compared with at least one previous spectrumacquired for the second reference material to check that the apparatusappears to be operating correctly. The spectrum acquired using thesecond reference material may be normalised using the respectivebackground spectrum acquired using the first reference material.

Where there are two reference material receiving locations, preferablythe beam switching arrangement has a third state:

the first state allowing light from the illumination arrangement toreach the sample receiving location and light reflected by a carriedsample to reach the collection optics;

the second state for redirecting light from the illumination arrangementtowards the first reference sample receiving location instead of thesample receiving location and for directing light reflected by areceived reference sample at the first reference sample receivinglocation towards the collection optics;the third state for redirecting light from the illumination arrangementtowards the second reference sample receiving location instead of thesample receiving location and for directing light reflected by areceived reference sample at the second reference sample receivinglocation towards the collection optics,thereby allowing selective detection at the detector of light reflectedby a carried sample, light reflected by a carried reference sample atthe first reference sample receiving location, and light reflected by acarried reference sample at the second reference sample receivinglocation.

The beam switching arrangement may comprise a roof mirror pair mountedfor movement between a first position corresponding to the first stateand a second position corresponding to the second state, and optionallya third position corresponding to the third state.

The roof mirror pair may be mounted for linear movement.

The beam switching arrangement may comprise a drive for driving the roofmirror pair. Thus the beam switching arrangement may comprise a drivefor driving the roof mirror pair in a linear movement direction.

The use of a roof mirror pair in the beam switching arrangement canallow the optical path length between the illumination arrangement/lightsource and the detector to be retained constant when the beam switchingarrangement is in the first state, the second state, and when present,the third state.

The beam switching arrangement may be arranged for scanning a receivedreference sample to allow taking of measurements over an extended areaof the reference sample. The scanning may be linear scanning.

The beam switching arrangement may be arranged to move the roof mirrorpair through a range of positions in which the roof mirror pair servesto redirect light from the illumination arrangement towards therespective reference sample receiving location instead of the samplereceiving location and directs light reflected by a received referencesample at the respective reference sample receiving location towards thecollection optics. In this way scanning may be achieved using the roofmirror pair.

Moreover scanning can be achieved simply and effectively using a singletransport mechanism both for switching between sample measurements andreference sample measurements and for scanning reference samples.Moreover the use of the roof mirror pair ensures that the optical pathlength between the illumination arrangement/light source and thedetector can be retained constant when in the different states andduring any scanning.

The diffuse reflectance spectroscopy apparatus may comprise a containerwithin which is disposed the, or both the sample receiving locations.This may be a housing of the apparatus or a separate enclosure.

The diffuse reflectance spectroscopy apparatus may comprise a memorywhich may store an internal reference correction spectrum whichrepresents a difference between a spectrum acquired using the apparatusfrom a sample placed at the sample receiving location and from the sameor an identical sample placed at the reference sample receivinglocation.

This can then be used in correcting the background spectrum measuredusing the first reference material.

According to a sixth aspect of the present invention there is provided amethod of manufacturing diffuse reflectance spectroscopy apparatus asdefined above.

The method may comprise the steps of:

placing a sample at the sample receiving location;

acquiring a first spectrum for the sample when at the sample receivinglocation;

placing the same or an identical sample at the reference samplereceiving location;

acquiring a second spectrum for the sample when at the reference samplereceiving location;

determining an internal reference correction spectrum from thedifferences between the first and second spectra for use in adjustingspectra determined from samples located at the reference samplereceiving location; and

storing the internal reference correction spectrum in a memory of thediffuse reflectance spectroscopy apparatus.

The diffuse reflectance spectroscopy apparatus may comprise aspectrometer.

The diffuse reflectance spectroscopy apparatus may comprise an accessoryfor use with a spectrometer.

In either case the spectrometer may, for example, be an FT-IR (FourierTransform InfraRed) spectrometer.

The diffuse reflectance spectroscopy apparatus may comprise an analysismodule for analysing measurements made by the detector in order toprovide compositional information concerning the sample. Alternativelyan analysis module may be provided separately from the reflectancespectroscopy apparatus.

In general the features of each of the aspects of the invention definedabove may be used with one another. Many practical embodiments willinclude a combination of these features. Further each of the optionalfeatures described above following any one of the aspects of theinvention can be used as optional features of each of the other aspectsof the invention, and could be rewritten with changes in wording asnecessary to correspond with the respective aspect of the invention.Such features are not repeated after each aspect of the invention in theinterest of brevity.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 schematically shows an FT-IR spectrometer embodying the presentinvention;

FIG. 2 schematically shows more detail of the optical arrangement of thespectrometer shown in FIG. 1;

FIG. 3 schematically shows the effect of changing the sample height ofthe operation of collection optics of the spectrometer shown in FIGS. 1and 2;

FIG. 4A illustrates a straight edge half beam block which may be used inthe spectrometer shown in FIGS. 1 and 2 as well as its effect on lightpassing through the system;

FIG. 4B shows a curved edged half beam block which may be used in thespectrometer of FIGS. 1 and 2 as well as its effect on light passingthrough the system;

FIG. 5A schematically shows how a filament of a light source may bepositioned relative to the acceptance field of an illuminationarrangement of the spectrometer shown in FIG. 1 and FIG. 2;

FIG. 5B shows a filament of a light source which is offset with respectto the acceptance field;

FIG. 6 schematically shows part of the spectrometer shown in FIGS. 1 and2 including a reference spectrum acquiring arrangement of thespectrometer;

FIG. 7A schematically shows a roof mirror of the reference spectrumacquiring arrangement shown in FIG. 6 in a first position in relation toa first reference sample to facilitate acquiring of a reference spectrumin respect of the first reference sample;

FIG. 7B shows the roof mirror shown in FIG. 7A in a second position withrespect to the first reference sample; and

FIG. 8 shows the roof mirror as shown in FIGS. 6, 7A and 7B in aposition where it facilitates acquiring a reference spectrum in respectof a second reference material.

FIG. 1 schematically shows an FT-IR (Fourier Transform Infra Red)spectrometer setup for use in diffuse reflectance spectroscopymeasurements. Thus the spectrometer shown in FIG. 1 constitutes diffusereflectance spectroscopy apparatus.

The spectrometer comprises a main housing 1 disposed on top of which isa sample receiving location 2. A sample receiving location 2 may, forexample, comprise a sample spinner for moving a carried sample 3relative to the main housing 1 of the spectrometer to allow measurementsto be taken over an extended area of the sample 3.

Within the housing 1 is provided an illumination arrangement 4 fordirecting light towards the sample 3. Further a collection systemcomprising collection optics 5 and a detector 6 is provided forreceiving light reflected from the sample 3. The light exits the housing1 and returns into the housing 1 via a window 11.

Also provided within the housing 1 of the spectrometer 1 is a referencespectrum acquiring arrangement 7, which will be described in more detailbelow, and a control unit 8 which is connected to and controls theoperation of the illumination arrangement 4, the collection system 5, 6and the reference spectrum acquiring arrangement 7. In the presentembodiment the control unit 8 comprises an analysis module 81 forreceiving data collected by the detector 6 and analysing this data inorder to provide information concerning the composition of a measuredsample 3. A memory module 82 is provided within the control unit forstoring data.

FIG. 2 shows the optical arrangement of the spectrometer of FIG. 1 inmore detail. In the present embodiment the illumination arrangement 4comprises a light source 41, an interferometer 42 and various opticalelements 43 a-43 g for guiding light from the source 41 to theinterferometer 42 and from the interferometer 42 towards the samplereceiving location 2. The illumination arrangement 4, and in particularthe optical elements 43 a-43 g, are arranged to focus the light source41 substantially at infinity. This leads to light from each point on thelight source 41 being spread across the field at the sample receivinglocation 2 which in turn will improve uniformity of illumination andconsistency in measurement. One might more naturally focus the lightsource 41 onto the sample receiving location 2. However this will leadto more spatial variation in illumination as an image of the lightsource 41 would then be formed at the sample.

In the present embodiment the light source 41 comprises an incandescentlamp having a filament 41 a. The illumination arrangement 4 is arrangedfor focusing the filament 41 a substantially at infinity. This ishelpful because the spectral content of the light emitted by the lamp 41will be dependent on the temperature of the filament according toPlanck's law of blackbody radiation. Generally speaking, the temperatureof the filament will not be constant along its length due to its shapeand/or variation in thicknesses.

Generally speaking, one can expect the lamp's emission to beapproximately equal in all directions and thus, by focusing the lamp atinfinity, it can be expected that light from each point of the sourcewill be spread evenly across the field at the sample location 2.

In the present embodiment the optical elements in the illuminationarrangement 4 include:

an off axis paraboloid mirror 43 a provided for collecting light fromthe source 41 and directing this towards the interferometer 42. Furtherthere is provided a flat mirror 43 b for reflecting light exiting theinterferometer 42 towards a second off axis paraboloid mirror 43 c,which feeds light towards an off axis ellipsoid mirror 43 d, which inturn reflects the light towards a first lens 43 e and onwards to asecond flat mirror 43 f, which reflects the light through a second lens43 g, after which light passes through the window 11 provided in thehousing 1 of the spectrometer to the sample receiving location 2.

In the present embodiment the source 41 is placed a small distancebeyond the focus of the first paraboloid mirror 43 a and thus the raysenter the interferometer 42 converging slightly.

The interferometer 42 is used in the present apparatus to modulate theillumination beam in order to encode the spectroscopic content of thelight in the conventional way for carrying out FT-IR measurements. Thusno further description of its operation is given here.

Any divergent rays within the interferometer 42 have a different pathlength than paraxial rays and thus will generate slightly frequencyshifted modulation. This will tend to detract from the spectralresolution of the system and needs to be controlled. Thus a J.stopaperture 44 is provided at the focus of the second paraboloid mirror 43c to block highly divergent rays. The first ellipsoid mirror 43 d formsan image of the J.stop aperture 44 at a focus between the ellipsoidmirror 43 d and the first lens 43 e. This provides a beam waist. Thefirst and second lenses 43 e and 43 g and the second flat mirror 43 fare used to relay the beam to a convenient sampling location whilstcontrolling its geometry and ensuring that an image of the source issubstantially at infinity.

The second paraboloid mirror 43 c and first ellipsoid mirror 43 d may bechosen to be complimentary so as to minimise aberrations. In particularthe first ellipsoid mirror 43 d may be chosen to correct for aberrationsin the second paraboloid mirror 43 c.

The lenses 43 e and 43 g of the illumination arrangement and the pathlengths are selected to give the system the following attributes:

-   -   1. A suitable beam diameter at the sample receiving location 2    -   2. A beam diameter which is roughly constant for a distance        above and below the nominal sample location 2.    -   3. Minimised divergence of the beam above and below the sample        location 2. This is useful as it enables the collection system        axis to be less inclined relative to the illumination axis which        in turn improves the system immunity to mis-positioning of the        sample 3.

It should be noted that whilst the main functionally important elementsof the illumination arrangement 4 are shown and described with referenceto FIG. 2, in a practical system they may well be further opticalelements in the path performing conventional functions.

As a whole there may be a relatively large number of optical elements inthe path between the source 41 and the sample location 2. In order topromote uniformity of illumination at the sample, the optical elements43 a to 43 f as well as components within the interferometer 42 arearranged so that any surface which may be non-uniform is spaced from animage (conjugate) of the sample. As an example, in the interferometer 42there is a beam splitter 42 a which can have a response that variessignificantly over its surface. Thus the optical system in the presentembodiment is arranged so that the conjugate of the sample is at areasonable distance from the beam splitter 42 a. In the presentembodiment the conjugate of the sample lies in the optical path betweenthe beam splitter 42 a and the first paraboloid mirror 43 a.

A possible problem when using FT-IR spectroscopy is that of the presenceof a double modulation artefact in measured spectra. Such an artefactcreates a false indication of short wavelength/high frequency energy inthe spectrum. The presence of such false energy in the spectrum has thepotential to influence chemometric predictions and therefore differencesin the magnitude of any such artefact between instruments can adverselyaffect the effectiveness of the calibration and the possibility ofreusing calibrations across different instruments.

Double modulation artefact is caused by light passing twice through theinterferometer 42. If light is allowed to pass twice through theinterferometer 42 it will be modulated on each pass and this will leadto unwanted and erroneous responses at the detector 6 if this lightpasses through the whole of the system. Thus in the present embodimenttwo half beam blocks are used in an effort to suppress any doublemodulation artefact.

As mentioned above the double modulation artefact comes about if lightis allowed to pass through the interferometer 42 twice. Generallyspeaking this can occur because when light enters the interferometer 42,half of the light will be reflected back out of the interferometer 42 inthe input direction, whilst half of the light travels forwards. Thismeans that there are reflective components before or after theinterferometer 42 there is an opportunity for the light to re-enter theinterferometer 42 and then re-emerge again.

One possibility to try to reduce the strength of double modulationartefact is to reduce the back reflection of all component parts in theoptical path or to otherwise try to redirect or block doubly modulatedlight from reaching the detector. This can be difficult. On the otherhand, the use of half beam blocks provides a particularly convenient wayto avoid light passing through the interferometer 42 twice and hencestop (or at least significantly reduce) receipt double modulated lightat the detector 6.

A half beam block consists of an obscuration placed across half of thebeam at a focus. Such a half beam block effectively stops (or at leastsignificantly reduces) the generation of double modulated light becausewhen light passes the focus in one half of the beam and is thenreflected back towards the beam block it will return on the oppositeside of the optical axis than it was when it passed the focus in theforward direction. Thus the half of the beam which can pass the beamblock in the forward direction will impact on the beam block whentravelling back in the reverse direction if it is reflected. Of courseon the other hand, the portion of the beam which meets the beam block inthe forward direction proceeds no further.

In the present embodiment a first half beam block 45 a is provided at afocus of the first paraboloid mirror 43 a just in front of the lightsource 41 and a second half beam block 45 b is provided at a focus ofthe first ellipsoid mirror 43 d. The first half beam block 45 a servesto block any light which exits the interferometer 42 in the reversedirection back towards the light source 41 from being reflected at thelight source 41 and progressing back towards the interferometer 42.

On the other hand, the second half beam block 45 b blocks the return tothe interferometer 42 of any light reflected by, say, lenses 43 e or 43g, the window 11 or the sample 3 or sample holder.

Each half beam block 45 a, 45 b is blackened to ensure that as far aspossible it does not reflect light.

A straight edged half beam block 45 a, 45 b, positioned halfway acrossthe beam or just over half way across the beam, will act tosignificantly reduce or remove any double modulation artefact byeffectively blocking the return of light which passes the half beamblock. However due to aberrations in the system, light which passes thehalf beam block and is returned may actually occupy a field which issmaller than half the beam. FIG. 4A illustrates this effect. FIG. 4Ashows (in the right hand half of the Figure) a straight edged half beamblock which might for example be used as the first half beam block 45 a.This is blackened and has a straight edge which can be seen down thecentre of the image shown in FIG. 4A. This half beam block, blocks halfof the beam such that half of the beam 450 may pass the beam block inthe forward direction. If this light is reflected and returns backtowards the half beam block then, due to aberrations in the system, theshape of the beam will have changed. An example of what a returned halfbeam looks like after reflection can be seen on the right hand half ofFIG. 4A and is labelled 451. Thus it will be seen that whilst near thecentre of the beam the returned beam is still close to the edge of thehalf beam block at regions away from this centre the edge of the beamhas retracted. Thus the dark area of the half beam block 45 a shownbetween the two beam portions 450 and 451 is not in fact performing auseful function. This part of the beam block 45 a is not needed to blockthe return beam. Thus it is possible to use a modified shape of halfbeam block in order to increase the amount of the beam which is allowedto pass the half beam block. This means that illumination efficiency canbe increased.

In FIG. 4B a curved edge half beam block 45 a′ is illustrated. This halfbeam block could be used in the same way as the straight edged half beamblock 45 a, or indeed the second half beam block 45 b. Here, by takinginto account the aberrations which occur in the system, the shape of thehalf beam block 45 a′ is chosen so that the beam 450′ which is allowedto pass the half beam block has a crescent shape. Again when the beam isreflected and returned its shape is changed by aberration. This isillustrated by the lighter region 451′ shown in the image of FIG. 4B.There is still a dark region between the forward beam 450′ and reflectedbeam 451′. Thus the half beam block 45 a′ is still functioning to blockreflected light and hence will prevent or at least significantly reduceany double modulation artefact but on the other hand, the amount oflight initially let through by the half beam block 45 a′ issignificantly increased.

Another technique may be used to further increase the efficiency of thesystem where a first half beam block 45 a is used in the region of thesource 41. This is to arrange the source 41 so that the filament 41 a isoffset with regard to the optical axis. FIG. 5A shows the filament 41Aaligned with the optical axis in a way which will allow the system tofunction. However in FIG. 5B the filament is shown to be offset in thedirection of the open half of the beam i.e. away from the side blockedby the half beam block 45 a. This means that more light from thefilament is allowed to enter the system rather than being directlyblocked by the half beam block 45 a.

Following the above description of the illumination arrangement 4, nowmore consideration is given to the collection system 5, 6.

Diffusely reflecting samples emit rays of light over a wide range ofangles. A traditional approach in diffuse reflectance spectroscopy is tocollect light over as much of this angular range as possible. This mightinvolve using an integrating sphere to collect the light. A particularproblem with such a system is that the change in the distance betweenthe sample and the entrance into the integrating sphere can producesignificant changes in measured spectra due to the angle of rays whichwill be collected and/or the intensity of light which will be collected.Furthermore in practice integrating spheres tend to be inhomogeneous inthemselves and there tend to be differences between one sphere and thenext. Thus this traditional type of arrangement is unsatisfactory wherethe aim is to provide a system which is tolerant to different conditionsand where different versions of the instrument are to behave assimilarly as possible.

As mentioned above, the illumination arrangement 4 is designed to have abeam diameter that is roughly constant for a distance above and below anominal sample location associated with the sample receiving location 2and also to minimise the divergence of the beam above and below thesample location. This helps towards the apparatus being insensitive tothe positioning or height of the sample 3 in relation to the apparatus.The collection optics 5 are similarly arranged with a view to provideinsensitivity to the height of the sample 3 in relation to the system.

This is important because, as illustrated for example in FIGS. 1 and 2,the sample 3 is likely to be provided at the sample receiving location 2in some kind of container such as a petri dish or sample bottle. Thelower wall of such a container through which the interrogating radiationmust pass to reach the sample 3 will tend to vary from container tocontainer. Thus the height of the sample 3 is likely to vary to somedegree.

FIG. 3 shows the collection optics 5 on a larger scale and alsoillustrates the effect of changing the sample height in the presentsystem. Note that in FIG. 3 only a simplified version of theillumination arrangement 4 is shown. The collection optics 5 comprise acollection off axis paraboloid mirror 51 and a second ellipsoid mirror52 which is chosen in order to correct aberrations in the paraboloid 51.Note however in an alternative configuration lenses might be used inplace of the mirrors 51, 52.

The collection optics 51, 52 are arranged to collect light reflectedfrom the sample 3 at the sample receiving location 2 over a small andwell defined range of angles and the collection optics 51, 52 are chosenso that the detector 6 is effectively focused at infinity. Further thecollection optics 5 and detector 6 are positioned so that the collectionsystem axis is inclined from the illumination beam direction, in thiscase from the vertical. This helps avoid collection of any specularreflection from the window 11 of the housing 1 and/or any samplecontainer. It also means that the reflected beam is separated from theillumination beam.

Because the detector 6 is focused at infinity, the active area of thedetector will limit the angular range of rays emitted by the samplewhich will be detected. This angular range will be small, typically inthe region of a few degrees.

The diameter of the collection paraboloid mirror 51 defines the entrancepupil of the system. It is selected to be sufficiently large toaccommodate all rays from the illuminated region of the sample 3 whichare emitted within the design angular acceptance range.

If the sample is displaced vertically, as illustrated by dotted lines inFIG. 3, this causes lateral displacement of the collection beam in thecollection optics 5. However, as the detector 6 is focused at infinity,such a displacement does not cause any change in the angular range ofthe rays collected. Since the entrance pupil is large enough toaccommodate this displacement (assuming that the vertical displacementis not beyond the operational range of the system), all of the rayswithin the design angular range are still routed to the detector 6.Moving the sample 3 has changed the routing of rays within thecollection optics 5 but has not changed the rays which are selected anddirected to the detector 6. Furthermore, since the mirrors are veryuniform reflectors, this change of routing of the rays should havelittle or no effect on the intensity or spectrum of the light incidenton the detector 6. Further, because the detector 6 is focused oninfinity, the rays from the sample reaching the detector will reach thesame area of the detector irrespective of the height of the sample(within operational range).

All of these factors help to provide uniformity in the performance ofthe system and between one instrument and another.

Thus within the working range of the system, the measured spectrum of asample should be independent of the sample height.

Note that in a typical system it would be normal to focus the detectorat the sample location but it has been realised by the presentapplicants that focusing the detector at infinity leads to theabove-mentioned advantages.

An intermediate focus position is available between the two mirrors 51and 52 and an aperture 53 is placed at this focus to guard against raysoutside the chosen angular range region reaching the detector 6. Theaperture 53 may be chosen so that the image of the aperture 53 at thedetector 6 is the same as, or slightly larger than the active area ofthe detector 6.

It should be noted that the sample 3 will emit light over a very broadrange of angles, although the collection optics 5 are designed to acceptonly reflections over a relatively narrow and predetermined range ofangles. Without the aperture 53, highly divergent light from the sample3 might strike the housing of the detector. Some of such rays mightreflect off the housing and strike the active area.

FIG. 6 shows part of the spectrometer of FIG. 1 from a direction whichis at right angles to the direction shown in FIG. 1. In the view shownin FIG. 6, the illumination arrangement 4, the sample receiving location2, the window 11 in the housing 1, and the reference spectrum acquiringarrangement 7 can be seen, however the collection system 5, 6 andcontrol unit 8 cannot be seen. The plane of illumination and collectionbeams is perpendicular to the plane of the paper with the deviceorientated as shown in FIG. 6.

FIG. 6 shows the reference spectrum acquiring arrangement 7 in moredetail than FIG. 1. The reference spectrum acquiring arrangementcomprises a roof mirror pair 71 mounted via a linear bearing 72 on alinear guide 73 and comprises a drive arrangement 76 for driving theroof mirror pair 71 relative to the linear guide 73 and hence relativeto the illumination arrangement 4 as well as the collection system 5, 6.The drive arrangement 76 comprises a motor 77 with a respective pulley77 a and a toothed belt 78 mounted on the motor pulley 77 a and an idlerpulley 79. The toothed belt 78 is attached to the linear bearing 72 totransfer drive to the roof mirror pair 71.

The reference spectrum acquiring arrangement 7 also comprises a firstreference sample receiving location 74 and a second reference samplereceiving location 75. The reference sample receiving locations 74, 75are provided on opposing sides of the plane of the illumination andcollection beams. In the present embodiment a first reference sample 74a is provided at the first reference receiving location 74 and a secondreference material 75 a is provided at the second reference materiallocation 75. In the present embodiment the first reference material 74 ais a highly reflective and generally featureless standard material suchas spectralon (a sintered PTFE material), or gold deposited on ascattering surface—“diffuse gold” whereas the second reference material75 a is another standard material which is stable but which has rathermore features in its infrared spectrum. Such a material might, forexample, be PVC, or a tablet made from a mixture of rare earth oxides.

The roof mirror pair 71 is mounted for linear movement in a directionwhich is perpendicular to the plane of the illumination and collectionbeams. The mirrors in the roof mirror pair 71 are set at right angles toone another. An apex where the mirrors meet or would meet is arranged atright angles to their linear movement direction. The mirrors projectdownwardly from this apex towards the first and second sample receivinglocations 74, 75.

The roof mirror pair 71 and its transport mechanism 72, 73 together actas a beam switching arrangement which is useable for selectivelyallowing measurement of spectra of samples carried at the samplereceiving location 2 on the one hand and an acquiring spectra from thefirst reference material 74 a and/or second reference material 75 a onthe other hand.

When the roof mirror pair 71 is in the location shown on solid lines inFIG. 6 it is displaced from the optical axis through the apparatus suchthat it has no effect on the delivery of light by the illuminationarrangement 4 and the collection of light by the collection system 5, 6.Thus with the roof mirror pair 71 in the position shown in FIG. 6, thespectrometer is able to operate as described above with the illuminationbeam leaving the illumination arrangement 4, progressing to the samplereceiving location 2 and impinging on any provided sample 3, beforebeing reflected by the sample 3 towards the collection system 5, 6.

However if the roof mirror pair 71 is moved along its linear guide 73 tothe position shown in dotted lines in FIG. 6, the roof mirror pair 71interrupts this path for light through the system. In particular, asshown again in dotted lines in FIG. 6, the illumination beam from theillumination arrangement 4 will be reflected via the roof mirror pair 71onto the first reference sample 74 a. Further, although not depicted inFIG. 6, light reflected by the first reference sample 74 a (at theappropriate acceptance angles) will be reflected by the roof mirror pair71 back towards the collection optics 5. Light is reflected first by oneof the mirrors in the pair and then by the respective other mirror inthe pair as it is reflected by the roof mirror pair in each direction.

The beam switching arrangement made up of the roof mirror pair 71 andits transport arrangement 72, 73 may be considered to have differentstates.

In a first state, as shown in solid lines in FIG. 6, the beam switchingarrangement allows the detector 6 of the collection arrangement tocollect light reflected from the sample 3 located at the “normal” samplereceiving location. However, the beam switching arrangement has otherstates which allow the detector 6 to detect light reflected from thefirst reference sample 74 a or the second reference sample 75 a as isdescribed in more detail below with reference to FIGS. 7A, 7B and FIG.8.

Furthermore, the roof mirror 71 and its transport arrangement 72, 73 maybe used for scanning the reference sample 74 a and/or 75 a to providespatial averaging when spectra are taken from these reference samples.

As illustrated in FIG. 7A, with the roof mirror pair 71 in a particularlocation, light may be fed to and reflected back from a first locationon the first reference material sample 74 a. As the roof mirror pair 71is moved in a linear direction (which is to the right in the orientationshown in FIGS. 7A and 7B) light will be fed to and reflected from adifferent portion of the first reference material 74 a. Thus by movingthe roof mirror pair 71 through a series of positions between thoseshown in 7A and 7B the illumination beam may be linearly scanned acrossthe first reference material 74 a such that an area of this referencematerial 74 a can be irradiated and light reflected therefrom can becollected for analysis.

Similarly, by moving the roof mirror pair 71 further in the samedirection (i.e. towards the right in the orientation shown in FIGS. 7A,7B and 8) the roof mirror pair 71 may be brought to another position, asshown in FIG. 8, where light from the illumination arrangement 4 is fedto the second reference material sample 75 a and the apparatus may beused for collecting spectra therefrom. Further it will be appreciatedthat the roof mirror pair 71 may be moved in such a way as to scan theillumination beam and collection area across the second referencematerial sample 75 a.

Thus the provision of the roof mirror pair 71 and a single lineartransport mechanism 72, 73 allows three way beam switching such that thesame illumination arrangement 4 and collection system 5, 6 may be usedwith the main sample receiving location 2 as well as two differentreference sample receiving locations 74 and 75. Furthermore this isachieved using only a single transport mechanism.

Further, the apparatus may be arranged so that the path length throughthe system, when light is being fed to and reflected from a sample atthe main sample receiving location 2, is the same as the path lengthwhen light is being fed to and reflected from a sample at the firstreference sample receiving location 74 and the second reference samplereceiving location 75. Furthermore this path length may be maintainedthe same as the beam is scanned across the first reference samplematerial 74 a and/or second reference sample material 75 a. Thisequality of the path length during switching and the scanning operationsis facilitated by using the roof mirror pair 71. This may be appreciatedfrom considering, for example, FIGS. 7A, 7B and 8. It can be noted, forexample, when comparing the optical paths in FIGS. 7A and 7B, that inFIG. 7A there is a greater horizontal component of the optical pathwhereas in FIG. 7B there is a greater vertical component in the opticalpath (in the orientation shown in FIGS. 7A and 7B).

The first and second reference material 74 a and 75 a may be retainedwithin an enclosure in the spectrometer. This may, for example, be themain housing 1 of the spectrometer.

This means that the reference sample materials 74 a, 75 a may beretained in their operative positions ensuring that they are protectedand removing the need for the user to present reference sample materialsat the main sample receiving location 2. This means that referencemeasurements are more likely to be taken and/or can be takenautomatically by the apparatus without intervention of the user.

The first reference sample material 74 a may be used to take abackground spectrum measurement from time to time to allow compensationfor changes in illumination which may occur from time to time due to,for example, changes in the output of the light source.

Such a background spectrum may be used for normalising other spectrataken of samples 3 presented at the main sample receiving location 2.

On the other hand the second reference sample 75 a may be used to enablean operational check to be made in respect of the spectrometer.

During manufacture, or at first use, a spectrum for the second referencematerial 75 a may be taken and stored in the memory module 82 in thecontrol unit 8. At a later time another spectrum for the same secondreference material 75 a may be taken and compared with that stored inthe memory module 82. If the later taken spectrum differs from thestored one by more than a threshold amount, this can be taken as anindication that the device has ceased to function properly. The devicemay be arranged to issue an alert if this condition is determined.

The memory module 82 may also store an internal reference correctionspectrum. This correction spectrum may be determined during manufactureand is representative of differences which will occur in the spectrumwhen a sample is interrogated when in position at one of the referencesample locations 74, 75 rather than at the main sample receivinglocation 2. This correction spectrum can be obtained during manufactureby presenting a sample at the main sample receiving location 2 andobtaining a spectrum and then obtaining a spectrum for the same sample(or an identical sample) positioned at one of the reference samplereceiving locations. From these two spectra the internal referencecorrection spectrum may be determined. This internal referencecorrection spectrum may then be used to further hone the backgroundspectrum which is acquired from the first reference sample material 74 afor use in correcting spectra obtained from “target” samples placed atthe main sample receiving location 2. The internal reference correctionspectrum allows the background spectrum to be adjusted to be closer towhat would have been measured if the first reference sample material wasplaced at the main sample receiving location 2.

Note that in alternatives rather than the diffuse reflectance apparatusbeing a complete spectrometer including everything from the light sourceto the analysis module, the diffuse reflectance apparatus may comprisesome subset of the parts. Thus, for example, the apparatus may be anaccessory or kit for use with an existing spectrometer. Typically anaccessory might comprise only part of the optics shown in FIG. 2. Adotted line box labelled A in FIG. 2 shows what an accessory maycomprise. Thus the accessory A may include only part of the illuminationarrangement 4 of the spectrometer described above. Although that partcan still be considered “an illumination arrangement”. The accessory Ain this case also includes the sample receiving location 2, thecollection system 5, 6 and the reference spectrum acquiring arrangement7 (although this is not shown in FIG. 2). Notably the light source 41and interferometer 42 are not included in this example accessory A andnor is the analysis module 81. These parts will be in theexisting/separate spectrometer for use with the accessory A. Theaccessory A may be provided as part of a kit including a beam block 45 ato mount near the source 41 and/or software to load into thespectrometer or its controlling computer.

The invention claimed is:
 1. A diffuse reflectance spectroscopyapparatus for use in analysing a sample comprising: a sample receivinglocation for receiving a sample for analysis; an illuminationarrangement for directing light from a light source towards a receivedsample; a detector for detecting light reflected by a received sample;collection optics for directing light reflected by a received sampletowards the detector; and a reference spectrum acquiring arrangementincluding at least one reference sample receiving location; and a beamswitching arrangement having first and second states, the first statefor causing or allowing light from the illumination arrangement to reachthe sample receiving location and light reflected by a carried sample toreach the collection optics; and the second state for causing orallowing light from the illumination arrangement to reach the referencesample receiving location instead of the sample receiving location andcausing or allowing light reflected by a received reference sample toreach the collection optics allowing selective detection at the detectorof light reflected by a carried sample and light reflected by a carriedreference sample, wherein the illumination arrangement comprises aninterferometer and a half beam block disposed substantially at a focusin an optical path between the interferometer and the light source forblocking light that exits the interferometer back towards the lightsource and is reflected by the light source from re-entering theinterferometer, and wherein the beam switching arrangement comprises aroof mirror pair mounted for movement between a first positioncorresponding to the first state and a second position corresponding tothe second state.
 2. The diffuse reflectance spectroscopy apparatusaccording to claim 1 in which the roof mirror pair is mounted for linearmovement.
 3. The diffuse reflectance spectroscopy apparatus according toclaim 1 in which the beam switching arrangement comprises a drive fordriving the roof mirror pair.
 4. The diffuse reflectance spectroscopyapparatus according to claim 1 in which the use of the roof mirror pairin the beam switching arrangement allows the optical path length betweenthe illumination arrangement/light source and the detector to beretained constant when the beam switching arrangement is in the firststate, and the second state.
 5. The diffuse reflectance spectroscopyapparatus according to claim 4 in which the beam switching arrangementis arranged for scanning a received reference sample to allow taking ofmeasurements over an extended area of the reference sample and the useof the roof mirror pair in the beam switching arrangement allows theoptical path length between the illumination arrangement/light sourceand the detector to be retained constant when the beam switchingarrangement is used in scanning.
 6. A diffuse reflectance spectroscopyapparatus for use in analysing a sample comprising: a sample receivinglocation for receiving a sample for analysis; an illuminationarrangement for directing light towards a received sample; a detectorfor detecting light reflected by a received sample; and collectionoptics for directing light reflected by a received sample towards thedetector, wherein the apparatus further comprises a reference spectrumacquiring arrangement comprising: at least one reference samplereceiving location; and a beam switching arrangement having first andsecond states, the first state for causing or allowing light from theillumination arrangement to reach the sample receiving location andlight reflected by a carried sample to reach the collection optics; andthe second state for causing or allowing light from the illuminationarrangement to reach the reference sample receiving location instead ofthe sample receiving location and causing or allowing light reflected bya received reference sample to reach the collection optics allowingselective detection at the detector of light reflected by a carriedsample and light reflected by a carried reference sample and in whichthe beam switching arrangement comprises a roof mirror pair mounted formovement between a first position corresponding to the first state and asecond position corresponding to the second state.
 7. The diffusereflectance spectroscopy apparatus according to claim 6 in which thereference spectrum acquiring arrangement comprises a reference sampledisposed at the reference sample receiving location.
 8. The diffusereflectance spectroscopy apparatus according to claim 6 in which thereference spectrum acquiring arrangement comprise two reference samplereceiving locations.
 9. The diffuse reflectance spectroscopy apparatusaccording to claim 6 in which a first of the reference sample receivinglocations holds a first reference material for use in acquiringbackground spectrum and a second of the reference sample receivinglocations holds a second reference material which is different from thefirst reference material and is for use in acquiring an operationalcheck spectrum.
 10. The diffuse reflectance spectroscopy apparatusaccording to claim 8 which is arranged to acquire a background spectrumfrom a reference sample held at the first reference sample receivinglocation and acquire an operational check spectrum from a referencesample held at the second reference sample receiving location.
 11. Thediffuse reflectance spectroscopy apparatus according to claim 8 in whichthe beam switching arrangement has a third state: the first state forcausing or allowing light from the illumination arrangement to reach thesample receiving location and light reflected by a carried sample toreach the collection optics; the second state for causing or allowinglight from the illumination arrangement to reach the first referencesample receiving location instead of the sample receiving location andfor causing or allowing light reflected by a received reference sampleat the first reference sample receiving location to reach the collectionoptics; the third state for causing or allowing light from theillumination arrangement to reach the second reference sample receivinglocation instead of the sample receiving location and causing orallowing light reflected by a received reference sample at the secondreference sample receiving location to reach the collection optics,thereby allowing selective detection at the detector of light reflectedby a carried sample, light reflected by a carried reference sample atthe first reference sample receiving location, and light reflected by acarried reference sample at the second reference sample receivinglocation.
 12. The diffuse reflectance spectroscopy apparatus accordingto claim 11 in which the roof mirror pair is mounted for movementbetween the first position corresponding to the first state and thesecond position corresponding to the second state, and a third positioncorresponding to the third state.
 13. The diffuse reflectancespectroscopy apparatus according to claim 6 in which the roof mirrorpair is mounted for linear movement.
 14. The diffuse reflectancespectroscopy apparatus according to claim 6 in which the beam switchingarrangement comprises a drive for driving the roof mirror pair.
 15. Thediffuse reflectance spectroscopy apparatus according to claim 6 in whichthe use of the roof mirror pair in the beam switching arrangement allowsthe optical path length between the illumination arrangement/lightsource and the detector to be retained constant when the beam switchingarrangement is in the first state and the second state.
 16. The diffusereflectance spectroscopy apparatus according to claim 6 in which thebeam switching arrangement is arranged for scanning a received referencesample to allow taking of measurements over an extended area of thereference sample.
 17. The diffuse reflectance spectroscopy apparatusaccording to claim 15 in which the beam switching arrangement isarranged for scanning a received reference sample to allow taking ofmeasurements over an extended area of the reference sample and the useof the roof mirror pair in the beam switching arrangement allows theoptical path length between the illumination arrangement/light sourceand the detector to be retained constant when the beam switchingarrangement is used in scanning.
 18. The diffuse reflectancespectroscopy apparatus according to claim 6 in which the diffusereflectance spectroscopy apparatus comprises a memory storing aninternal reference correction spectrum which represents a differencebetween a spectrum acquired using the apparatus from a sample placed atthe sample receiving location and from the same or an identical sampleplaced at the reference sample receiving location.
 19. A method ofmanufacturing a diffuse reflectance spectroscopy apparatus according toclaim 6, the method comprising the steps of: placing a sample at thesample receiving location; acquiring a first spectrum for the samplewhen at the sample receiving location; placing the same or an identicalsample at the reference sample receiving location; acquiring a secondspectrum for the sample when at the reference sample receiving location;determining an internal reference correction spectrum from thedifferences between the first and second spectra for use in adjustingspectra determined from samples located at the reference samplereceiving location; and storing the internal reference correctionspectrum in a memory of the diffuse reflectance spectroscopy apparatus.